Effect of natural populations of burrowing thalassinidean shrimp on sediment irrigation, benthic metabolism, nutrient fluxes and denitrification

The aim of this study was to determine the effect of burrowing by natural populations of Trypaea australiensis on benthic metabolism, nutrient flux, denitrification and irrigation rates using in situ benthic chambers and exclusion nets to minimise disturbance and associated artefacts. The burrows of T. australiensis enhanced total sediment porewater exchange rates 5-fold compared to uninhabited sediments. Calculated single-burrow flow rates were –1, and active burrow irrigation appeared to be linked to tidal movement; as such, ex situ studies (i.e. mesocosm or laboratory incubations) may underestimate burrow irrigation rates. The presence of T. australiensis increased sediment oxygen demand by 81% as compared to unoccupied sediments. Approximately 15% of this additional consumption was used by T. australiensis for respiration, with the remaining 85% consumed within the burrows by oxidation reactions and microbial respiration. The proportion of smaller sediment particles, porosity and organic matter content was increased in the presence of T. australiensis, and sediment surface chl a concentrations were ca. 50% lower than uninhabited sediments. Net denitrification rates were 4 times greater in the presence of active T. australiensis burrows and at densities of 22 burrows m^–2 were responsible for approximately 76% of total denitrification in the study site sediments. NH₄⁺ efflux was higher in the inhabited sediments and NO₃^– and NO₂^– effluxes were lower, suggesting that increased NH₄⁺ mineralisation may have enhanced denitrification via the close coupling of nitrification–denitrification in the inhabited sediments. As such, T. australiensis appears to contribute significantly to net sediment N₂ efflux in lower subtropical estuarine environments.